Local circuit neurones are key elements in the integrative processes in the brain that select and transform a particular sensory signal to produce a change in motor behavior. By choosing the most advantageous nervous system for study, the integrative processing in these small neurones can be followed through all its various steps and related directly to the behavior. Moreover, the neurones can be treated as identified individuals so that information on their patterns of connections, integrative properties and their individual contribution to the processing can be amassed over the course of many experiments. This proposal, for the study of local circuits that control leg movements in an insect, has two main objectives. First, to understand the contribution to these sensory-motor local circuits of three distinct types of local interneurons: spiking, nonspiking and neurosecretory interneurones. Second, to use this information to illustrate the design principles for the functioning of local circuits, and to understand why neurones with distinctive membrane properties are used for specific integrative tasks. The role will be examined of spiking local neurones in mapping sensory space, of nonspiking local interneurones and the compartmentalisation of function within them, in adjusting the gain of local reflexes, gating conflicting sensory signals and coordinating information from the different legs, and of local neurosecretory neurones in modulating the action of these circuits. Physiological methods of intracellular recording will be used to determine the membrane properties of selected neurones and the patterns of their connections. Morphological methods will reveal the shapes of individual neurones, the projections of their branches and the distribution of their synapses. Pharmacological methods will be used to determine the actions of the local neurosecretory neurones. Observations are to be made in an alert animal so that they can be related directly to behavior. The intent is to use these favorable neurones and circuits to provide insights into sensory-motor processing and thereby generate ideas that are applicable to the design and functioning of local circuit neurones in more complex brains.